We have studied the relationships among changes in cyclic AMP levels, myosin dephosphorylation, andmyosin light chain kinase phosphorylation during relaxation of tracheal smooth muscle (TSM). We have found that myosin is dephosphorylated when TSM are relaxed using various pharmacological agents. We have also found that myosin light chain kinase is phosphorylated only during cyclic AMP mediated relaxation of TSM.